This invention relates to a method of making optical waveguides and more particularly to a method of making multimode optical waveguides with diameter variations.
Waveguides used in optical communications systems are herein referred to as "optical waveguides" and are normally constructed from a transparent dielectric material such as glass or plastic.
An optical pulse launched into a multimode optical waveguide excites many modes, each traveling at a different group velocity. Such waveguides suffer from multimode dispersion. At the far end of the waveguide, the pulse is spread out in time by an amount that is proportional to the length of the waveguide because of the different group velocities of the modes. Such multimode dispersion can severely limit the information-carrying capacity of the waveguide.
It is known that multimode dispersion in optical waveguides can be reduced by deliberately enhancing coupling among the various modes in the waveguide. In accordance with the teachings of U.S. Pat. Nos. 3,666,348 Marcatili, 3,687,514 Miller and 3,912,478 Presby, mode coupling can be produced by causing variations in such waveguide parameters as core radius, core refractive index and waveguide axis. Since the optical power carried in the waveguide transfers back and forth between slow and fast modes, averaging takes place, so that the propagating light pulse travels at an average group delay, with a resultant reduction in pulse broadening. In the presence of such mode coupling the pulse width increases only as the square root of the waveguide length.
Several methods of fabrication have been suggested for making waveguides with these variations. In the aforementioned Miller et al and Marcatili patents, diameter variations are introduced by changing the velocity of drawing. The drawing velocity is changed by axially moving a member which supports the blank from which the waveguide is drawn.
In the aforementioned Presby patent, the drawing rate is changed to cause changes in the diameter by means of a variable air jet directed against the fiber.
U.S. Pat. No. 3,969,016 discloses the use of a corrugated roller to introduce corrugations into a waveguide to promote mode coupling.
In U.S. Pat. No. 3,909,110, Marcuse, variations in the index of refraction of the core are introduced by varying the doping during fabrication.
It is desirable to introduce diameter variations by changing an easily controlled parameter during the fabrication process.